[meta] generate API documentation

[chriseidhof]

Not sure yet what's the most robust way to do this. Definitely run jazzy, maybe in a pre-commit hook or something? That way we can just generate it in the docs folder (together with some high-level documentation maybe) and use github pages. Any ideas?

[MarcSteven]

https://github.com/SwiftDocOrg/swift-doc